CMOS image sensors use an ADC arrangement in which the voltage of each pixel is compared with a ramp voltage. At the point where the ramp voltage equals the pixel voltage, a comparator latches a digital count value into memory. In this architecture, the resolution of the ADC is directly related to the resolution and linearity of the ramp generation circuitry.
Known ramp generators using ADCs, switched capacitor integrators, charge pumps and current into a capacitor suffer from a number of problems. These problems include offsets, achieving the necessary gain within the time constants required for settling, area and power inefficiency, non-monotonicity and process/temperature dependence.